Communication system, method, base station, and communication device

ABSTRACT

A wireless communication system, using wireless base stations, and other devices, such as a relay node, interoperate with using spectrum aggregation and MIMO. Traffic usage is detected and based on channel utilization relative to capacity, spectrum aggregation is chosen over MIMO under certain conditions. On the other hand, under higher channel utilization system components switch to MIMO modes of operation to reduce demand on channel use, while providing good throughput for communications stations.

TECHNICAL FIELD

The present invention relates to a communication system, a communicationmethod, a base station, and a communication device.

BACKGROUND ART

A relay technique is standardized in IEEE (Institute of Electrical andElectronics Engineers) 802.16j. Further, in 3GPP (3rd GenerationPartnership Project) LTE-A (Long Term Evolution-Advanced) also, atechnique using a relay node (RN) is studied actively in order toimprove the throughput of a user equipment (UE) located at the celledge.

Further, in LTE, base stations are operated using a frequency band witha bandwidth of 1 MHz to 20 MHz with respect to a certain centerfrequency. It is thus not assumed that a communication device usesdiscrete channels. On the other hand, it is under consideration in LTE-Athat the user equipment reserves a band of 20 MHz or more by spectrumaggregation that makes simultaneous use of discrete or sequentialchannels to achieve higher throughput.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No.2006-148388

Japanese Unexamined Patent Application Publication No. 2006-148388 (JP2006-148388), attributable to the present inventor, and incorporatedherein by reference in its entirety, discloses a radio communicationdevice that includes a plurality of antennas, and uses some antennas forreception processing, for example, as a first reception/transmissionprocess and uses other antennas for transmission processing, forexample, as a second reception/transmission process.

SUMMARY OF INVENTION Technical Problem

In order to handle the dispersive channels by one receiver “branch” (anantenna, an analog processing unit etc., and sometimes referred to as areceiver “channel”), a filter or FFT compatible with a high bandwidth isnecessary. In view of this, it is possible to simplify the configurationof each branch by applying the radio communication device disclosed inthe above-described JP 2006-148388 to spectrum aggregation and makingdifferent channels (communication channels) correspond to the respectivebranches (receiver branches).

However, as recognized by the present inventor, the use of spectrumaggregation causes an increase in the number of channels allocated forcommunication with one communication device, which puts a severe strainon resources compared to multiple-input, multiple output (MIMO)communication.

In light of the foregoing, it is desirable to provide a communicationsystem, a base station and a communication device which are novel andimproved, and which enable switching between spectrum aggregation modeand MIMO mode in accordance with observed traffic volume for a givenchannel capacity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view showing a configuration of a communicationsystem according to an embodiment of the present invention.

FIG. 2 is an explanatory view showing an example of resource allocationin the case of using the same frequency in UL and DL.

FIG. 3 is an explanatory view showing an example of resource allocationin the case of using different frequencies in UL and DL.

FIG. 4 is an explanatory view showing an example of a format of DL radioframe.

FIG. 5 is an explanatory view showing an example of a format of UL radioframe.

FIG. 6 is an explanatory view showing a connection processing sequence.

FIG. 7 is an explanatory view showing an illustrative example of MBSFNtransmission/reception processing.

FIG. 8 is an explanatory view showing an example of frequency allocationin each cell.

FIG. 9 is a functional block diagram showing a configuration of a userequipment according to an embodiment of the present invention.

FIG. 10 is an explanatory view showing an illustrative example ofgrouping of channels.

FIG. 11 is a functional block diagram showing a configuration of a basestation according to an embodiment of the present invention.

FIG. 12 is an explanatory view showing an example of the degree ofcongestion of a channel group.

FIG. 13 is a sequence chart showing a flow for switching oftransmission/reception mode.

FIG. 14 is an explanatory view showing an illustrative example ofmulti-link connection of a relay node.

FIG. 15 is an explanatory view showing an illustrative example ofmulti-link connection of user equipment.

FIG. 16 is an explanatory view showing an example of a combination ofComp and spectrum aggregation.

FIG. 17 is an explanatory view showing relay transmission by a relaynode.

FIG. 18 is an explanatory view showing an example of a combination ofComp and spectrum aggregation.

FIG. 19 is an explanatory view showing relay transmission by a relaynode.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the appended drawings. Note that, in thisspecification and the appended drawings, structural elements that havesubstantially the same function and structure are denoted with the samereference numerals, and repeated explanation of these structuralelements is omitted.

Further, in this specification and the drawings, each of a plurality ofstructural elements having substantially the same function isdistinguished by affixing a different alphabetical letter to the samereference numeral in some cases. For example, a plurality of structuralelements having substantially the same function are distinguished likeuser equipments 20A, 20B and 20C where necessary. However, when there isno particular need to distinguish between a plurality of structuralelements having the same function, they are denoted by the samereference numeral. For example, when there is no particular need todistinguish between the user equipments 20A, 20B and 20C, they arereferred to simply as the user equipment 20.

Embodiments of the present invention will be described hereinafter inthe following order.

1. Basic Configuration of Communication System

(Example of Resource Allocation to Each Link)

(Example of Format of Radio Frame)

(Connection Processing Sequence)

(MBSFN)

(Example of Frequency Allocation to Each Cell)

2. Illustrative Configuration of Communication System

2-1. Switching Between Spectrum Aggregation Mode and MIMO Mode

2-2. Multi-Link Connection

2-3. Combination of Comp and Spectrum Aggregation

<1. Basic Configuration of Communication System>

A basic configuration of a communication system 1 according to anembodiment of the present invention is described hereinafter withreference to FIGS. 1 to 8. FIG. 1 is an explanatory view showing aconfiguration of the communication system 1 according to an embodimentof the present invention. Referring to FIG. 1, the communication system1 according to the embodiment of the present invention includes basestations 10A and 10B, a backbone network 12, user equipments 20A, 20Band 20X, and relay nodes 30A and 30B. The term “node” describesstations, devices, apparatuses, and equipment that relays a wirelesssignal from one device to another.

The base station 10 manages communication between the relay node 30 andthe user equipment 20 located inside a cell formed by the base station10. For example, the base station 10A manages scheduling information forcommunication with the user equipment 20X located inside the cell, andcommunicates with the user equipment 20X according to the schedulinginformation. Further, the base station 10A manages schedulinginformation for communication with the relay node 30A located inside thecell and scheduling information for communication between the relay node30A and the user equipment 20A.

Note that management of the scheduling information may be performed incooperation by the base station 10 and the relay node 30, may beperformed in cooperation by the base station 10, the relay node 30 andthe user equipment 20, or may be performed by the relay node 30.

The relay node 30 relays communication between the base station 10 andthe user equipment 20 according to the scheduling information managed bythe base station 10. Specifically, the relay node 30 receives a signaltransmitted from the base station 10 and transmits the amplified signalto the user equipment 20 by using frequency/time according to thescheduling information in the downlink. With such a relay in the relaynode 30, a signal-to-noise ratio is higher compared to directlytransmitting a signal from the base station 10 to the user equipment 20near the cell edge. A more detailed explanation of the relay node andhow it interoperates with a base station and user equipment is describedin JP Patent application 2010-040224, filed in the Japanese PatentOffice on Feb. 25, 2010, and in JP 2010-040227, filed in the JapanesePatent Office on Feb. 25, 2010, the entire contents of both of whichbeing incorporated herein by reference.

Likewise, in the uplink also, the relay node 30 relays a signaltransmitted from the user equipment 20 to the base station 10 accordingto the scheduling information managed by the base station 10, therebymaintaining a high signal-to-noise ratio. Although the case where onlythe relay node 30A exists in the cell formed by the base station 10A, aplurality of relay nodes 30 may exist in the cell formed by the basestation 10A.

Proposed as the types of the relay nodes 30 are Type 1 and Type 2. Therelay node 30 of Type 1 has an individual cell ID and is allowed tomanage its own cell. Thus, the relay node 30 of Type 1 operates in sucha way that it is recognized as the base station 10 by the user equipment20. However, the relay node 30 of Type 1 does not completely operateautonomously, and the relay node 30 performs relay communication withinthe range of resources allocated by the base station 10.

On the other hand, the relay node 30 of Type 2, differently from Type 1,does not have an individual cell ID and supports direct communicationbetween the base station 10 and the user equipment 20. For example, arelay transmission technique using cooperative relay or network codingis being studied. The following table 1 shows characteristics of Type 1and Type 2 under study.

TABLE 1 Item Type 1 Type 2 Decision R1-091098 R1-091632 Type of Relay L2and L3 Relay L2 PHY Cell ID Own cell ID No cell ID Transparency Nontransparent Relay Transparent Relay node to UE node to UE New cellCreate new cell Not create new cell (another eNB) RF parametersOptimized parameters N/A HO Inter cell HO HO transparently (generic HO)to UE Control Generate synch. channel, Not generate its own Channel RS,H-ARQ channel and channel but Generation scheduling informationdecodes/forwards donor etc. eNB′s signal to UE Backward Support (appearas a Re1-8 Support (able to relay also compatibility eNB to Rel-8 UE)to/from Rel-8 UE) LTE-A (Forward Support (it appear ?? compatibility)differently than Rel-8 eNB to LTE-A UE) Awareness to MS ?? (>Re1-8 eNBto LTE-A ?? UEs or Relay) Cooperation Inter cell cooperation Intra cellcooperation Backhaul Higher Lower utilization Usage model Coverageextension Throughput enhancement and coverage extension Cost HigherLower

The user equipment 20 communicates with the base station 10 directly orthrough the relay node 30 according to the scheduling informationmanaged by the base station 10. Data transmitted or received by the userequipment 20 may be voice data, music data such as music, lectures orradio programs static image data such as photographs, documents,pictures or charts, or video data such as movies, television programs,video programs, game images or the like. Further, the user equipment 20may be an information processing device having a radio communicationfunction such as a mobile phone or a personal computer (PC).

A management server 16 is connected to each base station 10 through thebackbone network 12. The management server 16 functions as a mobilemanagement entity (MME). Further, the management server 16 may functionas a serving gateway. The management server 16 receives managementinformation indicating the status of cell formed by each base station 10from the respective base stations 10 and controls communication in thecell formed by each base station 10 based on the management information.The function of the management server 16 may be incorporated into aplurality of physically separated structures in a distributed manner.

(Example of Resource Allocation to Each Link)

Resource allocation to each link is described hereinafter. In thefollowing description, a communication path between the base station 10and the relay node 30 is referred to as a relay link, a communicationpath between the relay node 30 and the user equipment 20 is referred toas an access link, and a direct communication path between the basestation 10 and the user equipment 20 is referred to as a direct link.Further, a communication path toward the base station 10 is referred toas UL (uplink), and a communication path toward the user equipment 20 isreferred to as DL (downlink). Communication in each link is based onOFDMA.

The relay node 30 separates the relay link and the access link byfrequency or time in order to avoid interference between the relay linkand the access link. For example, the relay node 30 may separate therelay link and the access link in the same direction by TDD (TimeDivision Duplexing) with use of a common frequency.

FIG. 2 is an explanatory view showing an example of resource allocationin the case of using the same frequency in UL and DL. Referring to FIG.2, one radio frame is made up of subframes 0 to 9. Further, in theexample shown in FIG. 2, the relay node 30 recognizes the subframes 8and 9 as resources for DL of the access link according to a directionfrom the base station 10 and therefore relays a signal transmitted fromthe base station 10 to the user equipment 20 with use of the subframes 8and 9.

Note that PSC (Primary Synchronization Channel) and SSC (SecondarySynchronization Channel), which are synchronous signals of the downlink,or PBCH (Physical Broadcast CHannel) is allocated to the subframes 0 and5. Further, a paging channel is allocated to the subframes 1 and 6.

FIG. 3 is an explanatory view showing an example of resource allocationin the case of using different frequencies in UL and DL. Referring toFIG. 3, a frequency f0 is used for DL, and a frequency f1 is used forUL. Further, in the example shown in FIG. 3, the relay node 30recognizes the subframes 6 to 8 of the frequency f0 as resources for DLof the access link according to a direction from the base station 10 andtherefore relays a signal transmitted from the base station 10 to theuser equipment 20 with use of the subframes 6 to 8 of the frequency f0.

Note that PSC and SSC, which are synchronous signals of the downlink,are allocated to the subframes 0 and 5 of the frequency f0 (for DL), anda paging channel is allocated to the subframes 4 and 9.

(Example of Format of Radio Frame)

Detailed examples of the frame format of DL radio frame and UL radioframe are described hereinafter with reference to FIGS. 4 and 5.

FIG. 4 is an explanatory view showing an example of the format of DLradio frame.

The DL radio frame is made up of subframes 0 to 9, each subframe is madeup of two 0.5 ms slots, and each 0.5 ms slot is made up of seven OFDM(Orthogonal Frequency Division Multiplexing) symbols.

As shown in FIG. 4, a control channel such as PCFICH (Physical ControlFormat Indicator CHannel), PHICH (Physical Hybrid ARQ Indicator CHannel)or PDCCH (Physical Downlink Control CHannel) is present in the first tothird OFDM symbols at the head of each subframe.

Each of the above channels contains the following information as anexample.

PCFICH: The number of symbols of PDCCH related to Layer 1 and Layer 2

PHICH: ACK/NACK for PUSCH

PDCCH: Downlink control information. Scheduling information (format suchas modulation scheme or coding rate) of PDSCH/PUSCH

Further, one resource block (1RB), which is a minimum unit of resourceallocation, is made up of six or seven OFDM symbols and 12 subcarriers.A demodulation reference (reference signal) is present in a part of theresource block.

Further, SSC, PBCH and PSC are present in the subframes 0 and 5. A freespace in the radio frame shown in FIG. 4 is used as PDSCH (PhysicalDownlink Shared CHannel).

FIG. 5 is an explanatory view showing an example of the format of ULradio frame.

Like the DL radio frame, the UL radio frame is made up of subframes 0 to9, each subframe is made up of two 0.5 ms slots, and each 0.5 ms slot ismade up of seven OFDM symbols.

As shown in FIG. 5, a demodulation reference (reference signal) ispresent in each of the 0.5 ms slots, and a CQI measurement reference ispresent in a distributed manner. The base station 10 or the relay node30 at the receiving end performs channel estimation by using thedemodulation reference and demodulates a received signal according tothe channel estimation result. Further, the base station 10 or the relaynode 30 at the receiving end measures the CQI measurement reference andthereby acquires CQI with the relay node 30 or the user equipment 20 atthe transmitting end.

Further, a free space in the radio frame shown in FIG. 5 is used asPUSCH (Physical Uplink Shared CHannel). Note that, upon receiving arequest for CQI report, the user equipment 20 or the relay node 30transmits the CQI report by using PUSCH.

(Connection Processing Sequence)

A connection processing sequence between the relay node 30 or the userequipment 20 and the base station 10 is described hereinafter withreference to FIG. 6.

FIG. 6 is an explanatory view showing a connection processing sequence.Referring to FIG. 6, the relay node 30 or the user equipment 20transmits RACH (Random Access CHannel) preamble to the base station 10(S62). Receiving the RACH preamble, the base station 10 acquires TA(Timing Advance) information and transmits the TA information togetherwith allocated resource information to the relay node 30 or the userequipment 20 (S64). For example, in the case where the transmissiontiming of the RACH preamble is known, the base station 10 may acquire adifference between the transmission timing and the reception timing ofthe RACH preamble as the TA information.

After that, the relay node 30 or the user equipment 20 transmits RRCconnection request to the base station 10 by using resources indicatedby the allocated resource information (S66). Receiving the RRCconnection request, the base station 10 transmits RRC connectionresolution indicating a transmission source of the RRC connectionrequest (S68). The relay node 30 or the user equipment 20 can therebyconfirm whether the base station 10 has received the RRC connectionrequest.

Then, the base station 10 transmits connection request indicating thatthe relay node 30 or the user equipment 20 is making a request forservice to the management server 16 that functions as MME (S70).Receiving the connection request, the management server 16 transmitsinformation to be set to the relay node 30 or the user equipment 20 asconnection setup (S72).

Then, the base station 10 transmits RRC connection setup to the relaynode 30 or the user equipment 20 based on the connection setup from themanagement server 16 (S74), and the relay node 30 or the user equipment20 makes connection setting. After that, the relay node 30 or the userequipment 20 transmits RRC connection complete indicating completion ofconnection setting to the base station 10 (S76).

Connection between the relay node 30 or the user equipment 20 and thebase station 10 is thereby completed, and communication becomesavailable. The above-described connection processing sequence is just byway of illustration, and the relay node 30 or the user equipment 20 andthe base station 10 may be connected by another sequence.

(MBSFN)

Hereinafter, MBSFN (Multimedia Broadcasting Single Frequency Network)transmission that is performed by the base station 10 and an exemplaryoperation of the relay node 30 in response to the MBSFN transmission aredescribed.

MBSFN is the mode where a plurality of base stations 10 simultaneouslytransmits data in a broadcast manner at the same frequency. Therefore,in MBSFN, the relay node 30 of Type 1 that virtually operates as a basestation transmits a control channel for DL or the like by using the samefrequency as that of the base station 10. A specific flow of MBSFNtransmission/reception processing is described hereinafter withreference to FIG. 7.

FIG. 7 is an explanatory view showing an illustrative example of MBSFNtransmission/reception processing. First, as shown in FIG. 7, the basestation 10 and the relay node 30 simultaneously transmit PDCCH. The basestation 10 transmits, after PDCCH, PDSCH for the user equipment 20 andR-PDCCH for controlling a relay. After R-PDCCH, the base station 10transmits PDSCH for the relay node 30 (relay target data). Anon-transmission period comes after PDSCH for the relay node 30.

The relay node 30 receives, after transmitting PDCCH, PDSCH (relaytarget data) from the base station 10 subsequent to a switching periodto reception processing. The relay node 30 then switches receptionprocessing to transmission processing in the non-transmission periodthat comes after PDSCH (relay target data) from the base station 10.Further, in the next step, the relay node 30 adds PDCCH to decoded PDSCH(relay target data) and then transmits the data to the user equipment20.

The existing user equipment that does not assume the existence of therelay node 30 can thereby make an advantage of the relay by the relaynode 30 without confusion.

(Example of Frequency Allocation to Each Cell)

An example of frequency allocation to each cell in the case where aplurality of cells are adjacent is described hereinafter.

FIG. 8 is an explanatory view showing an example of frequency allocationin each cell. In the case where each cell is made up of three sectors,frequencies f1 to f3 are allocated to the respective sectors as shown inFIG. 8, thereby suppressing the interference of frequencies at the cellboundary. Such allocation is particularly effective in a denselypopulated area with heavy traffic.

In LTE-A, in order to achieve end-to-end high-throughput, various noveltechniques such as spectrum aggregation, network MIMO, uplink multi-userMIMO and relay technique are being studied. Therefore, with the adventof high-throughput novel mobile applications, there is a possibilitythat exhaustion of frequency resources appears as an issue in a suburbanarea also. Further, in the introduction of LTE-A, it is highly possiblethat the installation of the relay node 30 will be activated for thepurpose of achieving infrastructure development at low costs.

<2. Illustrative Configuration of Communication System>

The basic configuration of the communication system 1 according to theembodiment is described above with reference to FIGS. 1 to 8.Hereinafter, an illustrative configuration of the communication system 1according to the embodiment is described.

(2-1. Switching Between Spectrum Aggregation Mode and MIMO Mode)

Recently, it has been studied that a communication device (the relaynode 30 or the user equipment 20) reserves a band of 20 MHz or more byspectrum aggregation that makes simultaneous use of discrete orsequential channels. However, in order to handle the dispersive channelsby one branch (transmission/reception resources such as an antenna andan analog processing unit), a filter or FFT compatible with a highbandwidth is necessary. Further, the use of spectrum aggregation causesan increase in the number of channels allocated for communication withone communication device, which would raise a concern that a severestrain is placed on resources compared to MIMO communication.

Against the above background, the communication system 1 according to anembodiment has been invented. According to the embodiment, it ispossible to simplify the configuration of each branch of a communicationdevice and selectively use spectrum aggregation mode and MIMO mode inaccordance with a traffic volume. Hereinafter, the user equipment 20 andthe base station 10 that constitute the communication system 1 accordingto the embodiment are described in detail. Moreover, more detailedexplanations of MIMO operation are provided in PCT InternationalPublication WO 2004/030238, and U.S. Pat. No. 6,862,271, the entirecontents of both of which being incorporated herein by reference.

FIG. 9 is a functional block diagram showing the configuration of theuser equipment 20 according to the embodiment of the present invention.Referring to FIG. 9, the user equipment 20 includes an analog processingunit 210, a digital processing unit 230, a control unit 242, and achannel selection unit 244. The analog processing unit 210 is made up ofa plurality of branches A, B and C.

Each branch includes an antenna 220 and a signal processing unit such asa BPF (Band-Pass Filter) 222, an AGC (Automatic Gain Control) 224, a DC(Down Converter)/UC (Up Converter) 226 and an AD/DA 228. The respectivebranches (transmission/reception resources) may include not onlyelements in the analog processing unit 210 but also elements in thedigital processing unit 230 such as FFT and IFFT.

The antenna 220 receives a radio signal from the base station 10 or therelay node 30 and acquires an electrical high-frequency received signal.Further, the antenna 220 transmits a radio signal to the base station 10or the relay node 30 based on a high-frequency transmission signalsupplied from the BPF 222.

The BPF 222 passes certain frequency components of a high-frequencyreceived signal acquired by the antenna 220. Further, the BPF 222 passescertain frequency components of a high-frequency transmission signalsupplied from the AGC 224. The AGC 224 makes automatic gain control of ahigh-frequency received signal supplied from the BPF 222 and ahigh-frequency transmission signal supplied from the DC/UC 226.

The DC/UC 226 down-converts a high-frequency received signal suppliedfrom the AGC 224 to a baseband received signal. Further, the DC/UC 226up-converts a baseband transmission signal supplied from the AD/DA 228to a high-frequency transmission signal.

The AD/DA 228 converts a baseband received signal supplied from theDC/UC 226 from analog to digital. Further, the AD/DA 228 converts abaseband transmission signal supplied from the digital processing unit230 from digital to analog.

The digital processing unit 230 includes an FFT that performs fastFourier transform on a baseband received signal supplied from eachbranch, a P (Parallel)/S (Serial), a demodulator, a decoder and so on asthe elements for reception. Further, the digital processing unit 230includes an encoder, a modulator, an S/P, an IFFT and so on as theelements for transmission, and supplies a baseband transmission signalon which a subcarrier signal is superimposed, for example, to the AD/DA228. Further, the digital processing unit 230 has MIMO processingfunction that enables MIMO communication.

The channel selection unit 244 selects channels (or channel group) to beused for communication in spectrum aggregation mode. Inappropriateselection can cause a problem in the following cases:

-   -   When a channel group to be processed by a certain branch exceeds        the limits of the capacity of the branch; i.e., when channels in        the channel group to be processed are too dispersed.    -   When there is a large difference in propagation path        characteristics between channels in a channel group to be        processed by a certain branch, and the expected improvement in        throughput is not achieved.

Therefore, the channel selection unit 244 makes channel selection by thefollowing procedure:

(1) Acquire information (use channel information) such as the centerfrequency and the bandwidth of the respective channels used by theconnected base station 10

(2) Determine resources (data rate) to be reserved for the userequipment 20

(3) Classify a plurality of channels used by the connected base station10 as a channel group

(4) Determine a combination of a channel group and a branch.

Specifically, in the above (3), the channel selection unit 244classifies one or more than one channels which can be simultaneouslyprocessed by each branch as a group according to a settable centerfrequency, filter size, FFT size or the like. For example, the channelselection unit 244 classifies channels into groups in such a way thatthe bandwidth of each group does not exceed the bandwidth that can behandled by the FFT. Grouping of channels is specifically describedhereinafter with reference to FIG. 10.

FIG. 10 is an explanatory view showing an illustrative example ofgrouping of channels. In the example shown in FIG. 10, use channels ofthe base station 10 are O, P, Q, R, S, T, U and so on. In this case, thechannel selection unit 244 classifies the channels O, P and Q as achannel group #1, classifies the channels R, S and T as a channel group#2, and classifies the channel U as a channel group #3, for example.

Further, regarding the above (4), each branch performs signal processingon known signals (e.g. reference signals) of all channels transmittedfrom the base station 10. Then, the channel selection unit 244 averagesout the communication quality such as reception level or SINR ofchannels constituting each channel group and thereby acquires thecommunication quality of each channel group with respect to each branch.For example, the channel selection unit 244 averages out thecommunication quality of the channels O, P and Q by the branch A andthereby acquires the communication quality of the channel group #1.

Further, the channel selection unit 244 combines each branch and achannel group with the highest communication quality in each branch. Forexample, the channel selection unit 244 combines the branch A and thechannel group #1, combines the branch B and the channel group #2, andcombines the branch C and the channel group #3. Note that if one channelgroup has the highest communication quality in different branches, thechannel selection unit 244 may combine the branch in which thecommunication quality of the channel group is higher and the channelgroup. Further, the channel selection unit 244 may combine the otherbranch and the channel group with the second highest communicationquality in the other branch.

Referring back to FIG. 9, the configuration of the user equipment 20 isfurther described hereinafter. The control unit 242 of the userequipment 20 controls the overall operation in the user equipment 20,such as transmission processing, reception processing, and connectionprocessing with the relay node 30 or the base station 10. For example,the control unit 242 performs TPC (Transmit Power Control), CQI reporttransmission control or the like.

Further, the control unit 242 requests the base station 10 to use thechannel group selected by the channel selection unit 244 (i.e. thechannel group combined with the branch). Although a method of therequest is not particularly limited, exemplary methods are as follows

-   -   Acquire channels for use by autonomously making a connection        request by a given slot (RACH: Random Access CHannel) with        respect to each selected channels. The connection request may be        made from the branch combined with the channel.    -   Notify the selected channels by using one channel, not with        respect to each selected channels. One channel may be any one of        the selected channels or another channel. Further, one channel        may be transmitted from the base station 10 through a broadcast        channel such as PBCH or operating parameter information of an        adjacent base station for handover. Further, the notification        may be contained in any transmission signal (e.g. RACH) in a        connection processing sequence (Call set up).

Based on the above request, the base station 10 allocates resources inthe channels to the user equipment 20, and the base station 10 and theuser equipment 20 can thereby communicate in the spectrum aggregationmode. For example, the base station 10 can transmit radio signals byusing the channel groups #1 to #3, the user equipment 20 performsreception processing of the radio signal using the channel group #1 bythe branch A, performs reception processing of the radio signal usingthe channel group #2 by the branch B, and performs reception processingof the radio signal using the channel group #3 by the branch C.

Note that the channel selection unit 244 and the control unit 242 mayexecute the selection of the channel group and the use request to thebase station 10 as described above according to a command from the basestation 10. Further, the configuration of the user equipment 20 isapplicable also to the relay node 30. Specifically, the relay node 30may include a plurality of branches which respectively transmit andreceive signals using different channels to thereby realize spectrumaggregation. At this time, the relay node 30 may select a channel groupwith the suitable communication quality and request the base station 10to use the selected channel group by the above-described method.

Hereinafter, the configuration of the base station 10 is described withreference to FIG. 11.

FIG. 11 is a functional block diagram showing the configuration of thebase station 10 according to the embodiment of the present invention.Referring to FIG. 11, the base station 10 includes an analog processingunit 110, a digital processing unit 130 and a control unit 142. Further,the analog processing unit 110 is made up of a plurality of branches A,B and C.

Each branch includes an antenna 120 and a signal processing unit such asa BPF 122, an AGC 124, a DC/UC 126 and an AD/DA 128. The respectivebranches may include not only elements in the analog processing unit 110but also elements in the digital processing unit 130 such as FFT andIFFT. Further, although the base station 10 includes three branches inthe example shown in FIG. 11, the number of branches in the base station10 is not particularly limited.

The antenna 120 receives a radio signal from the user equipment 20 orthe relay node 30 and acquires an electrical high-frequency receivedsignal. Further, the antenna 120 transmits a radio signal to the userequipment 20 or the relay node 30 based on a high-frequency transmissionsignal supplied from the BPF 122.

The BPF 122 passes certain frequency components of a high-frequencyreceived signal acquired by the antenna 120. Further, the BPF 122 passescertain frequency components of a high-frequency transmission signalsupplied from the AGC 124. The AGC 124 makes automatic gain control of ahigh-frequency received signal supplied from the BPF 122 and ahigh-frequency transmission signal supplied from the DC/UC 126.

The DC/UC 126 down-converts a high-frequency received signal suppliedfrom the AGC 124 to a baseband received signal. Further, the DC/UC 126up-converts a baseband transmission signal supplied from the AD/DA 128to a high-frequency transmission signal.

The AD/DA 128 converts a baseband received signal supplied from theDC/UC 126 from analog to digital. Further, the AD/DA 128 converts abaseband transmission signal supplied from the digital processing unit130 from digital to analog.

The digital processing unit 130 includes an FFT that performs fastFourier transform on a baseband received signal supplied from eachbranch, a P/S, a demodulator, a decoder and so on as the elements forreception. Further, the digital processing unit 130 includes an encoder,a modulator, an S/P, an IFFT and so on as the elements for transmission,and supplies a baseband transmission signal on which a subcarrier signalis superimposed, for example, to the AD/DA 128. Further, the digitalprocessing unit 130 has MIMO processing function that enables MIMOcommunication.

The control unit 142 controls the overall communication in the cellformed by the base station 10, such as transmission processing,reception processing, connection processing with the relay node 30 orthe user equipment 20, and management of scheduling information. Forexample, when use (connection) of a plurality of channels is requestedfrom the relay node 30 or the user equipment 20, the control unit 142may execute a connection processing sequence with the relay node 30 orthe user equipment 20 and make scheduling of a resource block in therequested channels to the relay node 30 or the user equipment 20. Thebase station 10 can thereby realize spectrum aggregation that uses aplurality of channels requested from the relay node 30 or the userequipment 20. Note that, in spectrum aggregation, the control unit 142may associate each of the requested channels to any branch of the basestation 10 and communicate with the relay node 30 or the user equipment20 by using the associated branch.

Further, the control unit 142 functions as a mode control unit thatswitches the spectrum aggregation mode to MIMO mode in accordance withthe degree of congestion (traffic volume) of channels being used forspectrum aggregation. The mode switching is described herein below. Moregenerally, when the channel usage (e.g., traffic volume, SNIR level, %of channel capacity, error rate, spectral occupancy, number of users,reserved etc.) is detected to be above a certain level the control unit142 switches to the MIMO mode of operation.

As a communication technique for improving throughput, MIMO is usedbesides spectrum aggregation. MIMO is a technique that transmits aplurality of signal sequences in parallel from a plurality oftransmission antennas, receives them with a plurality of receptionsantennas, and separates the plurality of signal sequences by using theindependence of propagation path characteristics between the pluralityof transmission antennas and the plurality of receptions antennas.

However, in MIMO, there is a case where the independence of propagationpath characteristics between the plurality of transmission antennas andthe plurality of receptions antennas is low, and high throughput isunachievable in this case. On the other hand, in spectrum aggregation,throughput increases with the number of channels. Therefore, highthroughput is more reliably achieved by spectrum aggregation than MIMO.

In view of the above, the control unit 142 gives a higher priority tothe operation in the spectrum aggregation mode. This ensures higherthroughput. On the other hand, when the traffic volume increases and thedegree of congestion (or usage) becomes higher, it is desirable toreduce the occupied bandwidth per user. Therefore, the control unit 142switches the spectrum aggregation mode to the MIMO mode according to anincrease in the traffic volume.

For example, in the case where the channel groups #1 to #3 are used forspectrum aggregation communication with the relay node 30 or the userequipment 20 and when the degree of congestion of the channel groups #1and #2 exceeds a threshold as shown in FIG. 12, the control unit 142switches the spectrum aggregation mode to the MIMO mode. The controlunit 142 may use the channel group #3 with the degree of congestionbelow the threshold in the MIMO mode. Note that the degree of congestionmay be the absolute traffic volume in each channel group or the resourceusage rate in each channel group. Further, the threshold of the degreeof congestion may be different among channel groups.

Further, when the degree of congestion of a certain channel groupexceeds the threshold but the degree of congestion of a plurality ofchannel groups remains below the threshold, the control unit 142 maycontinue to operate in the spectrum aggregation mode by excluding thecertain channel group with the degree of congestion exceeding thethreshold. For example, when the degree of congestion of only thechannel group #1 exceeds the threshold, the control unit 142 maycontinue to perform spectrum aggregation by using the channel groups #2and #3.

Further, upon switching to the MIMO mode, the control unit 142 maytransmit trigger information for notifying (prompting) the switching tothe relay node 30 or the user equipment 20 which is the other end ofcommunication by spectrum aggregation. For example, the control unit 142may transmit the trigger information with use of PDCCH or PDSCH.Further, the trigger information may contain channel information (centerfrequency, bandwidth etc.) used for MIMO communication or informationindicating switching timing.

The control unit 242 of the relay node 30 or the user equipment 20 canthereby switch the transmission/reception mode of the analog processingunit 210 and/or the digital processing unit 230 to the MIMO mode basedon the trigger information. Note that the analog processing unit 210and/or the digital processing unit 230 waits for receiving a MIMOpreamble upon switching to the MIMO mode.

The configurations of the user equipment 20 and the base station 10 aredescribed above. Hereinafter, a series of flow for switching of thetransmission/reception mode is described with reference to FIG. 13.

FIG. 13 is a sequence chart showing a flow for switching of thetransmission/reception mode. First, the user equipment 20 acquiresinformation of a plurality of channels used by the base station 10 inresponse to a command from the base station 10, for example (S404).After that, the user equipment 20 classifies the plurality of channelsused by the base station 10 into channel groups (S408). Specifically,the user equipment 20 classifies one or more than one channels which canbe simultaneously processed by each branch as one channel group.

Then, the user equipment 20 determines a combination of each branch anda channel group with the highest communication quality in each branch(S412), and requests the base station 10 to use the channel group whosecombination with the branch is determined (S416). After that, the basestation 10 performs connection processing with the user equipment 20 andallocates a resource block in the requested channels to the userequipment 20, and the base station 10 and the user equipment 20 canthereby perform data communication by spectrum aggregation (S420).

After that, when the degree of congestion of the traffic in the cellexceeds a certain criterion (YES in S424), the base station 10 transmitstrigger information indicating switching from the spectrum aggregationmode to the MIMO mode to the user equipment 20 (S428). Based on thetrigger information, the user equipment 20 switches thetransmission/reception mode to the MIMO mode and then performs datacommunication with the base station 10 by MIMO (S432). Note that, whenthe congestion of the traffic in the cell is reduced, the base station10 may give a command to execute the processing from S404 to the userequipment 20 for switching to the spectrum aggregation mode.

As described above, when operating in the spectrum aggregation mode, thebase station 10 switches the transmission/reception mode to the MIMOmode according to an increase in the traffic volume. It is thus possibleto ensure high throughput by the spectrum aggregation mode when thetraffic volume is low, and reduce the occupied bandwidth per user by theMIMO mode when the traffic volume increases.

(2-2. Multi-Link Connection)

When there are more available resources in another base station 10 thanin the connected base station 10, the relay node 30 can switch the relaylink to that base station 10 to thereby make effective use of theresources.

However, if a connection processing sequence (Call set up) with anotherbase station 10 is performed each time switching the relay link, aswitching delay due to the multiprocedure occurs. In light of this, therelay node 30 according to the embodiment creates multi-link connectionwith a plurality of base stations 10 with use of a plurality of branchesto thereby reduce the switching delay. This is described in detailbelow.

First, the relay node 30 acquires use channel information (centerfrequency, bandwidth etc.) of a plurality of connectable base stations10. Then, the relay node 30 makes Call set up with the plurality ofconnectable base stations 10 and completes the procedure up to RRCconnection complete. The relay node 30 is thereby in multi-linkconnection with the plurality of base stations 10.

FIG. 14 is an explanatory view showing an illustrative example ofmulti-link connection of the relay node 30. In the example shown in FIG.14, the relay node 30 is in multi-link connection with the base station10A and the base station 10B. Note that the relay node 30 can make Callset up in parallel with use of a plurality of branches (or links). Forexample, as shown in FIG. 14, the relay node 30 may simultaneously makeCall set up with the base station 10A with use of the branch A (or linkA) and Call set up with the base station 10B with use of the branch B(or link B).

After that, the relay node 30 uses the relay link with the highest gainamong the relay links with the plurality of base stations 10. Forexample, if the gain of the relay link with the base station 10A ishigher than the gain of the relay link with the base station 10B in theexample shown in FIG. 14, the relay node 30 selects use of the relaylink with the base station 10A. Specifically, the relay node 30 relayscommunication related to the user equipment 20 by using the relay linkwith the base station 10A, and sets the relay link with the base station10B as a standby link.

The base station 10 may add a specifier that specifies whether the relaylink with the connected relay node 30 is a standby link or not to aninterface (S1-MME IF) between the management server 16 (MME) and thebase station 10. For example, in the example shown in FIG. 14, the basestation 10B may add the specifier that specifies that the relay linkwith the relay node 30 is a standby link to the interface with themanagement server 16. The management server 16 or the base station 10can thereby perform processing according to whether each relay link is astandby link or not. For example, the management server 16 or the basestation 10 may give a higher priority to scheduling of the relay linkwhich is not a standby link, and may approve a request related to astandby link if there are available resources. Further, the relay node30 may use different branches for different links. For example, therelay node 30 may use the branch A for the relay link with the basestation 10A, use the branch B for the relay link with the base station10B, and use the branch N (or link N) for the access link with the userequipment 20.

After that, when the necessity for reducing the traffic or allocatingresources or the like arises, the relay node 30 selects use of thestandby link. For example, the relay node 30 may switch the relay linkfor use from the relay link with the base station 10A to the relay linkwith the base station 10B. Note that the relay node 30 may takenecessary steps for obtaining resources related to the relay link withthe base station 10B while relaying communication related to the userequipment 20 by using the relay link with the base station 10A. Forexample, the relay node 30 may make advance contact with the managementserver 16 through the base station 10 about resources intended to beobtained (resources likely to be switched). It is thereby expected toinstantaneously respond to a request for obtaining resources from therelay node 30.

By the above manner, it is possible to reduce the delay time from therise of the necessity to switch the relay link to the switching of therelay link. The same procedure is applicable to the access link also.Specifically, the user equipment 20 can reduce the switching delay ofthe access link by creating multi-link connection with a plurality ofconnectable relay nodes 30.

FIG. 15 is an explanatory view showing an illustrative example ofmulti-link connection of the user equipment 20. In the example shown inFIG. 15, the relay node 30A is connected with the base station 10A, therelay node 30B is connected with the base station 10B, and the userequipment 20 is in multi-link connection with the relay node 30A and therelay node 30B. In this case, the user equipment 20 can switch theaccess link for use between the access link with the relay node 30A andthe access link with the relay node 30B according to need.

(2-3. Combination of Comp and Spectrum Aggregation)

Recently, CoMP (Coordinated Multipoint Transmission) has been studied asa technique of improving the link tolerance with respect to userequipment existing at the cell edge. Comp is a technique in which aplurality of adjacent base stations simultaneously transmit the samesignal with use of the same channel. An embodiment that combines Compand spectrum aggregation is described hereinafter.

EXAMPLE 1

In this example, when the traffic of a certain base station 10 iscongested and a plurality of channels are not allocable to one userequipment 20, the base station 10 transmits a signal to the userequipment 20 by using one channel and, simultaneously, a nearby basestation 10 transmits a signal to the user equipment 20 by using adifferent channel. Then, the relay node 30 receives the signals that aretransmitted from a plurality of base stations 10 using differentchannels and transmits them to the user equipment 20. It is therebypossible to improve the throughput of communication related to the userequipment 20. This is specifically described hereinafter with referenceto FIGS. 16 and 17.

FIG. 16 is an explanatory view showing an example of a combination ofComp and spectrum aggregation. As shown in FIG. 16, the base station 10Atransmits a signal to the user equipment 20 by using f1 and,simultaneously, the base station 10B and the base station 10C transmitsignals to the user equipment 20 by using f5 and f9, respectively.

Then, the relay node 30 receives the signals transmitted from therespective base stations 10 and transmits them to the user equipment 20.The relay node 30 may perform communication with the respective basestations 10 by using different branches. For example, the relay node 30may communicate with the base station 10A by using the branch A,communicate with the base station 10B by using the branch B, andcommunicate with the base station 10C by using the branch C.

Further, although the relay node 30 receives signals to the userequipment 20 from the respective base stations 10 with discrete channelsin the frequency domain, the relay node 30 relays the signals to theuser equipment 20 by using less dispersed channels. For example, whenthe relay node 30 receives signals with channels f1, f5 and f9 which arediscrete in the frequency domain as shown in FIG. 17, the relay node 30may relay the signals to the user equipment 20 by using channels f4, f5and f6 which are consecutive in the frequency domain. Because the userequipment 20 can thereby receive the signals with the channels f4, f5and f6 which are consecutive in the frequency domain, it is possible toreduce the processing load of the user equipment 20.

Although the case where the number of channels for transmission is thesame as the number of channels for reception is shown in FIG. 17, thenumber of channels for transmission may be smaller than the number ofchannels for reception. For example, the number of channels forreception may be three, and the number of channels for transmission maybe two. In this case, a coding rate in the channels for transmission maybe set higher than a coding rate in the channels for reception. Further,the number of channels for transmission may be one.

Further, a method of selecting a channel to be used for transmission isnot particularly limited. For example, a channel to be used fortransmission may be selected from channels close to the frequency bandin which high SINR is obtained with the user equipment 20.

EXAMPLE 2

In this example, the base station 10 transmits signals to the userequipment 20 belonging thereto with a plurality of channels by spectrumaggregation. Then, a nearby base station 10 also transmits a signal inthe channel with a large attenuation among the signals in the pluralityof channels transmitted from the base station 10, and the relay node 30relays the signal to the user equipment 20. It is thereby possible tostrengthen the signal transmitted from the base station 10. This isspecifically described hereinafter with reference to FIGS. 18 and 19.

FIG. 18 is an explanatory view showing an example of a combination ofComp and spectrum aggregation. As shown in FIG. 18, the base station 10Atransmits signals to the user equipment 20 by using f1, f3 and f6. Ifthe attenuation of f3 and f6 is large (when needed SNIR becomes notsatisfied), the base station 10B and the base station 10C also transmitthe signals that are transmitted by the base station 10A by using f3 andf6.

For example, the signal transmitted by the base station 10A with use off3 is transmitted also by the base station 10B with use of f2, and thesignal transmitted by the base station 10A with use of f6 is alsotransmitted by the base station 10C with use of f7. Then, as shown inFIG. 19, the relay node 30 transmits the signal received from the basestation 10B with f2 to the user equipment 20 with use of f3 andtransmits the signal received from the base station 10C with f7 to theuser equipment 20 with use of f6. In this configuration, it is possibleto strengthen the signals transmitted from the base station 10A by usingf3 and f6. Although the case where the base stations 10B and 10Cstrengthen the signals by using frequencies different from that of thebase station 10A is described above, the base stations 10B and 10C maystrengthen the signals by using the same frequency as that of the basestation 10A. For example, the base station 10B may use f3, and the basestation 10C may use f6.

Further, in both the example 1 and the example 2, the management server16 (MME/Serving Gateway) that monitors the respective links among thebase station 10, the relay node 30 and the user equipment 20 serves animportant role. Further, in the example 1, information for makingspectrum aggregation between a plurality of adjacent base stations 10 incooperation with each other is transmitted and received by the X2interface between the base stations 10 and the S1 interface between thebase station 10 and the management server 16. The information may bechannel measurement report list with respect to each channel used forspectrum aggregation, position information and capability (bandwidththat can be transmitted and received at a time etc.) of the relay node30 or the user equipment 20, extra resource information of each basestation 10 or the like. In the example 2 also, information forstrengthening spectrum aggregation channels is transmitted and receivedby the X2 interface and the S1 interface.

Although preferred embodiments of the present invention are described indetail above with reference to the appended drawings, the presentinvention is not limited thereto. It should be understood by thoseskilled in the art that various modifications, combinations,sub-combinations and alterations may occur depending on designrequirements and other factors insofar as they are within the scope ofthe appended claims or the equivalents thereof.

For example, it is not always necessary to execute the respective stepsin the processing of the communication system 1 in this specification inchronological order according to the sequence shown in the sequencecharts. For example, the respective steps in the processing of thecommunication system 1 may be executed in the sequence different fromthe sequence shown in the sequence charts or may be executed inparallel.

Furthermore, it is possible to create a computer program that causeshardware such as a CPU, ROM and RAM incorporated in the base station 10,the user equipment 20 and the relay node 30 to function equally to therespective elements of the base station 10, the user equipment 20 andthe relay node 30 described above. Further, a memory medium that storessuch a computer program may be provided.

REFERENCE SIGNS LIST

10A, 10A, 10B, 10C Base station

20 User equipment

30, 30A, 30B Relay node

110, 210 Analog processing unit

130, 230 Digital processing unit

142, 242 Control unit

244 Channel selection unit

The invention claimed is:
 1. A wireless communication stationcomprising: a plurality of antennas configured to receive and transmitwireless signals between the wireless communication station and at leastone other device; a transceiver coupled to said plurality of antennas,said transceiver being MIMO compatible and spectrum aggregationcompatible; and a controller that is triggered to switch between a MIMOmode of operation and a spectrum aggregation mode of operation, based ona level of detected channel usage.
 2. The communication station of claim1, wherein said transceiver is a multichannel transceiver, each channelin said multichannel transceiver having a separate analog processingsection.
 3. The communication station of claim 1, wherein said wirelesscommunication station is a base station.
 4. The communication station ofclaim 3, wherein said base station further includes a detectorconfigured to detect the level of channel usage and prepare an indicatorthat is transmitted to another device to trigger the another device tooperate in the MIMO mode of operation or the spectrum aggregation modeof operation based on the level of channel usage detected.
 5. Thecommunication station of claim 1, wherein said wireless communicationstation is a relay node.
 6. The communication station of claim 5,wherein said relay node is configured to receive a signal, thatindicates whether the controller is to switch between the MIMO mode ofoperation and the spectrum aggregation mode of operation based on thelevel of channel usage detected by at another device.
 7. Thecommunication station of claim 1, wherein said controller includes ausage detector that triggers a mode change from said spectrumaggregation mode of operation to said MIMO mode of operation when saidlevel of channel usage is above a predetermined threshold.
 8. Thecommunication station of claim 1, wherein said wireless communicationstation is a relay node, said relay node being configured to communicatewith different base stations using different communication modes ofoperation than a communication mode used by the relay node tocommunicate with user equipment.
 9. The communication station of claim8, wherein said transceiver is configured to receive a plurality ofdifferent transmissions from a plurality of base stations and relayinformation in said plurality of transmissions to a single userequipment over a single communication link.
 10. The communicationstation of claim 8, wherein said relay node is further configured toreceive signals from a first base station using spectrum aggregation andcombine information from said first base station with informationreceived from a second base station via a different communication link,and forward the information from the first base station with theinformation from the second base station in one signal to said userequipment.
 11. The communication station of claim 1, wherein saidcontroller is configured to make a request of a base station to assign achannel group from within a plurality of different candidatecommunication channels.
 12. A method for exchanging information in awireless communication system comprising: receiving signals throughmultiple antennas from a plurality of different signal sources;combining said signals and forming an aggregate set of information;forming a transmission signal including said aggregate set ofinformation; checking for an indication of channel usage being above apredetermined level, and transmitting said aggregate set of informationvia MIMO transmission when said usage is detected in a communicationchannel as being above said predetermined level, and transmitting saidaggregate set of information in a spectrum aggregation transmission modewhen said usage is detected as being below said predetermined level. 13.The method of claim 12, further comprising: detecting the level ofchannel usage within said communication channel and comparing said levelof channel usage against the predetermined level; and generating atrigger signal to trigger a switch between a MIMO mode of operation anda spectrum aggregation mode of operation, depending on a level of usagedetected in said detecting step.
 14. The method of claim 12, furthercomprising: relaying information in said aggregate set of information toa user equipment.
 15. The method of claim 14, wherein said relaying stepincludes using a different signaling protocol than a signaling protocolused with signals received by the relay node.
 16. A non-transitorycomputer readable medium having instructions that when executed by aprocessor implement steps of: receiving signals through multipleantennas from a plurality of different signal sources; combining saidsignals and forming an aggregate set of information; forming atransmission signal including said aggregate set of information;checking for an indication of channel usage being above a predeterminedlevel, and transmitting said aggregate set of information via MIMOtransmission when said usage is detected in a communication channel asbeing above said predetermined level, and transmitting said aggregateset of information in a spectrum aggregation transmission mode when saidusage is detected as being below said predetermined level.
 17. Awireless communication station comprising: a plurality of antennasconfigured to receive and transmit wireless signals between the wirelesscommunication station and at least one other device; a transceivercoupled to said plurality of antennas, said transceiver being MIMOcompatible and spectrum aggregation compatible; and means for switchingbetween a MIMO mode of operation and a spectrum aggregation mode ofoperation based on a level of detected channel usage.
 18. Thecommunication station of claim 1, further comprising: a plurality offilters configured to perform band limitation, wherein a number of theplurality of filters is equal to a number of the plurality of antennas.19. The communication station of claim 18, wherein the plurality ofantennas includes three antennas.